"It's mainly about sending a message -- about the drills and also its anger over the recent UN rights report."

A UN-mandated commission published a damning report earlier this month, detailing horrific human rights abuses in North Korea and concluding that they could comprise crimes against humanity.

Despite the start of the South Korea-US drills on Monday, which the North routinely condemns as rehearsals for invasion, relations between Seoul and Pyongyang are currently enjoying something of a thaw.

This year's drills overlapped with the end of the first reunion for more than three years of families divided by the Korean War -- an event that has raised hopes of greater cross-border cooperation.

Pyongyang had initially insisted that the joint exercises be postponed until after the reunions finished on Tuesday. But Seoul refused and -- in a rare concession -- the North allowed the family gatherings on its territory to go ahead as scheduled.

Dongguk University's Kim said North Korea was experienced at calibrating its military provocations, and would probably stop at the short-range missile tests.

"I don't think it will take any further steps and risk escalating tensions. Pyongyang is more interested in seeing some benefit from its compromise on the reunions," he said.

The launches came days after an incursion by a North Korean patrol boat across the disputed Yellow Sea border that has been the scene of brief but bloody naval clashes in the past.

No shots were fired and the vessel retreated to its side of the boundary after repeated warnings from the South Korean navy.

North Korea fired half-a-dozen short-range missiles into the Sea of Japan over several days in May last year, just as the Korean peninsula was emerging from an unusually protracted surge in military tensions that followed the North's third nuclear test.

North Korea has hundreds of short-range missiles and has developed and tested -- with limited success -- several intermediate-range models.

Its claims to have a working inter-continental ballistic missile have been treated with scepticism by most experts, but there is no doubt that it is pushing ahead with an active, ambitious missile development programme.